In a cellular system of related technology, an antenna array of a base station is usually provided with horizontally arranged arrays as shown in FIG. 1 and FIG. 2. A beam at an emitting end of the base station can only be adjusted in the horizontal direction, while the beam is provided with a fixed downtilt in the vertical direction; hence, various beam-forming techniques or pre-coding techniques are based on channel state information in the horizontal direction. However, since wireless signals are transmitted in three-dimensional way in space, optimum system performance cannot be achieved with fixed downtilt.
Beam adjustment in the vertical direction is meaningful to reduction of inter-cell interference and improvement of system performance. As antenna technology develops, in the industry, each oscillator of an active antenna can be controlled independently, as shown in FIG. 3 and FIG. 4. With such three-dimensional antenna array, signals transmitted by the base station can be beam-formed for UE both in the horizontal direction and in the vertical direction. Such antenna array enables dynamic adjustment of beam in the vertical direction.
For accurately orienting the beam to the UE in the vertical direction and thereby achieving maximum beam-forming gain, one common way for the base station to determine a beam-forming vector in the vertical direction requires the UE to feed back channel state information (CSI) in the vertical direction. In another way, the UE is configured with multiple CSI feedback configurations adopting different vertical beam-forming vectors respectively, the UE performs CSI feedback based on an optimum CSI feedback configuration and informs the base station of corresponding location information (i.e., informs the base station of a particular vertical beam-forming vector corresponding to the optimum CSI feedback configuration), such that the base station may utilize an optimum vertical beam-forming vector to perform vertical beam-forming.
The maximum beam-forming gain is detailed as follows.
Firstly, the base station determines N non-zero power channel state information reference signal (NZP CSI-RS) resources. A quantity of ports of each CSI-RS resource is identical to a quantity of groups of antenna elements. Each port of one CSI-RS resource corresponds to one group of antenna elements, for example, a first port corresponds to a first column of vertical antennas, a second port corresponds to a second column of vertical antennas, and so forth.
The base station determines one beam-forming weight vector for each CSI-RS resource, where the beam-forming weight vector can be determined based on a vertical angle to be covered by the CSI-RS resource. For each port of the CSI-RS resource, a pilot signal at the port is weighted by the beam-forming vector and then transmitted from one group of antenna elements corresponding to the port. For example as shown in FIG. 5, there are 16 antenna elements. Four antenna elements in the vertical direction are classified into one group. There are four groups of antenna elements and each group includes four antenna elements. Each group of antenna elements is used to transmit a pilot signal at one port of the CSI-RS resource. A pilot signal Sn(i) at an i-th port is weighted by a beam-forming weight vector [Wn(0)Wn(1)Wn(2) Wn(3)]T and then transmitted from an i-th group of antenna elements, i.e., an i-th column of antennas. In FIG. 5, the subscript n is for distinguishing CSI-RS resources. Beams for various CSI-RS resources have different directions, usually, three different beam-forming weight vectors [Wn(0)Wn(1)Wn(2)Wn(3)]T with n equals 0, 1 and 2 are configured and the UE may perform measurements based on three CSI-RS resources corresponding to the three beam-forming weight vectors. The UE may report CSI measured on a CSI-RS resource having an optimum channel quality and location information of the CSI-RS resource in all configured CSI-RS resources. The base station can obtain a current optimum vertical beam-forming weight vector based on the location information and perform vertical beam-forming using the current optimum vertical beam-forming weight vector.
In practical, the UE may report location information and CSI corresponding to multiple optimum CSI feedback configurations and the base station selects one vertical beam or several different vertical beams for downlink data transmission. Here, the UE may further need to report a quantity of CSI feedback configurations corresponding to the currently reported CSI and location information, i.e., a total quantity of pieces of CSI that are fed back, or the UE may further need to report a quantity of CSI feedback configurations proposed for the base station.
A CSI feedback of the UE may be a periodic CSI feedback using a physical uplink control channel (PUCCH) or an aperiodic CSI feedback using a physical uplink shared channel (PUSCH). In the periodic CSI feedback, the base station configures a periodic PUCCH resource and the UE reports corresponding CSI on the configured resource periodically. In the aperiodic CSI feedback, the base station triggers the UE to perform CSI feedback via downlink control information (DCI) and the UE performs CSI feedback on a PUSCH in an uplink subframe corresponding to a triggered subframe. A CSI process set is pre-configured for the UE via a high layer signaling and the triggering process is achieved using the DCI. By configuring multiple CSI process sets, the UE can feed back pieces of CSI corresponding to multiple CSI-RS that are beam-formed by different vertical beam-forming vectors.
In the aperiodic CSI feedback, when triggering terminals to perform CSI feedback, the base station needs to transmit different control signalings for different terminals, and each terminal performs CSI feedback after receiving the control signaling for the terminal. However, this approach leads to large consumption of downlink signalings from the base station.